The formation of seals between fittings has been the subject of much attention because a seal lacking integrity can lead to failure of the device of which it is an element. In one attempt at solving the problems inherent in seals, Wood, in U.S. Pat. No. 4,643,467, provides a threaded tubing seal having a tapered thread with a sealing profile and a shoulder seal, as well as a plastic, self-energized O-ring seal. Blose et al. describes a metal to metal seal, having engageable threads of relatively long pitch disposed on a relatively steep taper in U.S. Pat. No. 3,224,799. In U.S. Pat. No. 5,106,130, Ellsworth and Baer disclose a thread coupling for reinforced pipe where the female and male threads are ground or scribed into the ends of the pipes to be joined where at least one of the scribed threads has a paced thread contact molded onto it.
In an attempt to create a coupling for non-ferrous pipe, Carter, Sr. teaches the formation of male and female coupling elements where the male element terminates in a cylindrical non-threaded, non-tapering, sealing surface adjacent its threads and the female element includes an internal sealing sleeve or liner, in U.S. Pat. No. 4,537,426. Pierpont, Jr. attempts to provide an integral joint thread on fiberglass pipe by molding threaded portions having an outer surface which is corrugated or irregular and which tapers down to the inside diameter of the pipe in U.S. Pat. No. 3,661,670. A fiberglass reinforced pipe with threaded ends is disclosed by Kimura et al. in U.S. Pat. No. 4,893,658, where metal plating is formed on the surface of at least one of the threaded ends in an effort to create a better seal. Valdes describes a synthetic plastic pipe coupling where the internally threaded fitting is made of a material harder than the material of the pipe which is coupled with the fitting, so that the threads of the fitting act as a die for creation of helical grooves on the pipe, in U.S. Pat. No. 4,488,738. In U.S. Pat. No. 5,015,014, Sweeney discloses a plastic pipe section having annular rabbets in the outer and inner peripheral surfaces thereof which are tapered substantially at a common angle so that two sections can form a male/female joint therebetween.
Unfortunately, the prior art does not adequately address the problems faced when a pipe having a standard NPT or NPTF male connection is threaded into a plastic female connection in either an adjacent pipe or the port of a fluid and/or air handling device made of plastic in order to create joint integrity without the disadvantages of over-tightening, especially where pre-applied sealants are used. Such a system is highly desired and is provided by the present invention.